1. Field of the Invention
The invention relates to lasers, and particularly high repetition rate, line narrowed excimer and molecular fluorine lasers for operation at specified pulse energies.
2. Discussion of the Related Art
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems operating around 248 nm, as well as the following generation of ArF-excimer laser systems operating around 193 nm. Vacuum UV (VUV) lithography uses the molecular fluorine (F2) laser operating around 157 nm.
Higher energy, higher efficiency excimer and molecular fluorine lasers are being developed as lithographic exposure tools for producing very small structures as chip manufacturing proceeds deeper into the sub-0.18 micron regime. Specific characteristics of laser systems sought to be improved upon particularly for the lithography market include higher repetition rates, increased energy stability and dose control, increased lifetimes of optical components, increased percentage of laser system operation xe2x80x9cuptimexe2x80x9d, narrower output emission linewidths, improved wavelength and bandwidth calibration and stability, and improved compatibility with stepper/scanner imaging systems.
It is important for their respective applications to the field of sub-quarter micron silicon processing that each of the above laser systems become capable of emitting a narrow spectral band of known bandwidth and around a very precisely determined and finely adjustable absolute wavelength. Techniques for reducing bandwidths by special resonator designs to less than 100 pm for use with all-reflective optical imaging systems, and for catadioptric imaging systems to less than 0.6 pm, are being continuously improved upon.
A line-narrowed excimer or molecular fluorine laser used for microlithography provides an output beam with specified narrow spectral bandwidth. Narrowing of the bandwidth is generally achieved through the use of a bandwidth narrowing and/or wavelength selection and wavelength tuning module (hereinafter xe2x80x9cline-narrowing modulexe2x80x9d) including most commonly prisms, diffraction gratings and, in some cases, optical etalons. The line-narrowing module typically functions to disperse incoming light angularly such that light rays of the beam with different wavelengths are reflected at different angles. Only those rays fitting into a certain xe2x80x9cacceptancexe2x80x9d angle of the resonator undergo further amplification, and eventually contribute to the output of the laser system.
Parameters of the line-narrowing module such as the magnitude of angular dispersion, reflectivity for specific wavelengths, linearity (i.e. absence of wavefront distortions), scattering of the beam, etc., will thus affect the performance of the laser. The optical components, i.e., prisms, gratings, etalons, etc., of the line-narrowing module undergo changes that reduce their performance with prolonged energy doses of laser power. This reduced performance of the line-narrowing module is attributed to xe2x80x9cagingxe2x80x9d of the optical components, wherein particularly the optical surfaces take on defects resulting in reduced efficiency.
Line-narrowed excimer and molecular fluorine lasers are particularly required to emit pulses of predetermined energy for precise industrial processing. The energy of the output laser beam depends on the composition and quality of the gas mixture, the input energy to the discharge and the efficiency of the laser resonator including the line-narrowing optics. The gas mixture is typically replenished online using careful monitoring and gas control procedures and the input energy to the discharge is processor-controlled typically using an expert control system. The initial efficiency of the resonator can be optimized by careful selection and arrangement of line-narrowing components. However, as the optical components and the laser tube itself age, the efficiency of the laser decreases steadily from its initial optimum state.
It is recognized herein that the pulse energy can be maintained at the required level, when laser efficiency is reduced by aging, by increasing the driving discharge voltage, as illustrated at FIG. 1. The range xcex94E represents the pulse energy range within which the operating laser is required to emit laser pulses (the extent of xcex94E is exaggerated for illustration in FIG. 1). The range xcex94HV represents the voltage range within which the input discharge voltage may be adjusted in order to produce the desired pulse energies. An absolute limitation of xcex94HV is imposed by physical constraints on the components of the gas discharge laser, while a practical limitation may be imposed by laser performance considerations (see U.S. patent application No. 60/171,717, which is assigned to the same assignee and is hereby incorporated by reference).
Referring to FIG. 1, curve 1 represents the energy versus driving voltage curve for a laser having a new line-narrowing module and laser tube, or a line-narrowing module and laser tube wherein the components, particularly the optics, have not yet aged. Curves 2-4 are energy versus driving voltage curves for laser systems having line-narrowing modules at various aging states from less aging to more aging.
When components such as the optics and the laser tube are new, and curve 1 is the relevant E-V curve, the driving voltage can be adjusted to as low as HVmin and the desired pulse energies are achieved. The highest available pulse energy of the laser system is Emax, as shown, wherein the discharge voltage is set at HVmax and the laser is operating with new optics according to curve 1, even though Emax would lie outside the acceptable range of pulse emission energies. When the optics are aged such the laser is operating according to curve 2, application of only HVmin to the discharge will no longer achieve the desired pulse energies. However, by increasing the input voltage, pulse energies in the desired range xcex94E are easily achieved. At a further stage of aging of the optics, the laser operates along curve 3 when only input voltages at or near HVmax will produce the desired pulse energies. When the optics have aged further such the laser is operating according to curve 4, the desired pulse energies are no longer achievable.
Before the aging stage is reached wherein the laser operates according to curve 4, the line-narrowing module is typically replaced with a new one so that the laser can again operate according to curve 1, and the process repeats itself. The duration between when the laser operates along curve 1 with new optics and when the laser can no longer produce pulse energies in the desired range xcex94E is known as the xe2x80x9clifetimexe2x80x9d of the line-narrowing module, or the lifetime of the optics. At some point, the laser tube itself ages such that even with a new line-narrowing module, the laser cannot produce the desired pulse energies. At this point, the laser tube has reached the end of its xe2x80x9clifetimexe2x80x9d.
Replacing the line-narrowing module implies servicing and can result in undesirable service cost and downtime of the laser system. It is desired to increase the lifetime of the line-narrowing module and optics.
It is therefore an object of the invention to provide an excimer or molecular fluorine laser system having a line-narrowing module and/or other optical components with increased lifetimes over conventional systems.
In accord with the above object, a laser system, particularly an excimer or molecular fluorine laser system, includes a laser tube filled with a gas mixture and having a plurality of electrodes therein connected to a discharge circuit for energizing the gas mixture, and a resonator including a line-narrowing or line-selection module for generating a line-narrowed laser beam. The laser system includes an attenuator module for attenuating the pulse energy of the laser beam. The preferred attenuator module is disposed outside of the resonator for reducing the pulse energy of the output beam.
The laser system is preferably configured such that the operating discharge voltage range xcex94HV is higher than for conventional systems, such that the discharge circuit applies higher energy electrical pulses to the discharge than conventional laser systems, and particularly such that when the line-narrowing module is new, the minimum achievable pulse energy is above a desired pulse energy, i.e., when the discharge voltage is set to HVmin. The energy pulses output from the laser resonator are attenuated by the attenuator module to the desired energy.
The attenuator module is preferably configured to exhibit variable attenuation, and is particularly preferred to exhibit such variable attenuation in steps. Thus, as the optics of the line-narrowing module and/or the laser tube age, the attenuation is reduced accordingly so that the laser system continues to achieve the desired pulse energies.
The attenuator module may comprise a series of partially reflective plates, preferably each being arranged at Brewster""s angle, each having a different reflectivity coefficient. The attenuator module is adjustable so that one of the plates may be selected that corresponds to the attenuation desired depending on the aging stage of the line-narrowing module and/or the laser tube.
The attenuator module may comprise one or more plates each having a coating for reflecting a unique percentage of incident light of a particular polarization, such as xcfx80-polarized light. The resonator of the laser is configured to output a substantially polarized beam, such as exhibiting xcfx80-polarization upwards of 95%-97%. The xcfx80-polarization of the beam may be produced by using Brewster surfaces such as the windows on the laser tube or a prism surface or otherwise. The polarization of the beam may be produced using a polarization plate in the resonator.
Advantageously, the effective lifetime of the line-narrowing module and/or the laser tube is increased by using a high attenuation when the line-narrowing module and/or laser tube is new, and a decreasing attenuation percentage as the line-narrowing optics and/or laser tube age. Further, operation at higher energies than is typically desired is possible when the line-narrowing optics are particularly new, such as by reducing the attenuation percentage.